This invention relates to gas turbine combustors; and, in particular, to improvements in gas turbine combustors for the further reduction of air pollutants such as nitrogen oxides (NOx).
In an effort to reduce the amount of NOx in the exhaust gas of a gas turbine, inventors Wilkes and Hilt devised the dual stage, dual mode combustor which is disclosed in U.S. Pat. No. 4,292,801 issued Oct. 6, 1981 to a common assignee of the present invention, and incorporated herein by reference. In this patent, it is disclosed that the amount of exhaust NOx can be greatly reduced, as compared with a conventional single stage, single fuel nozzle combustor, if two combustion chambers are provided. The specific configuration as described in the above identified patent includes an annular array of primary nozzles each of which discharges fuel into the primary combustion chamber, and a central secondary nozzle which discharges fuel into the secondary combustion chamber. The secondary nozzle has an axial fuel delivery pipe surrounded at its discharge end by an air swirler which provides combustion air to the fuel nozzle discharge.
The combustor is operated by first introducing fuel and air into the first chamber for burning therein. Thereafter, the flow of fuel is shifted into the second chamber until burning in the first chamber terminates, followed by a reshifting of fuel distribution into the first chamber for mixing purposes, with burning occurring only in the second chamber. The combustion in the second chamber is rapidly quenched by the introduction of substantial amounts of dilution air into the downstream end of the second chamber to reduce the residence time of the products of combustion at NOx producing temperatures thereby providing a motive force for the turbine section which is characterized by low amounts of NOx, carbon monoxide and unburned hydrocarbon emissions.
Further development in this area produced a two stage (diffusion/premixing) secondary fuel nozzle as described in commonly assigned, application Ser. No. 07/501,439 filed Nov. 25, 1986, now U.S. Pat. No. 4,982,570 the disclosure of which is also expressly incorporated by reference herein. As described in the above identified co-pending application, it was discovered that further reduction in the production of NOx could be achieved by altering the design of the central or secondary nozzle such that it may be described as a diffusion piloted premixed nozzle. In operation, a relatively small amount of fuel is used to sustain a diffusion pilot, while a premix section of the nozzle provides additional fuel for ignition of the main fuel supply from the upstream primary nozzles.
The primary object of this invention is to improve transfer to premixed mode of operation via a diffusion flame, and once in the premixed operation mode, to turn off the diffusion flame and start the premixing flame so as to enable the gas turbine to operate at its design point for any desired length of time. Transferring to a premixed mode with a diffusion flame in accordance with this invention has the characteristic of low combustion dynamic pressure activity.
In accordance with an exemplary embodiment of the invention, a two stage (diffusion/premixing), gas only secondary fuel nozzle is provided which has two fuel circuits. This allows the nozzle to operate in a premixed mode or diffusion mode. The secondary nozzle of each combustor is located within a centerbody and extends through a liner provided with a swirler through which combustion air is introduced for mixing with fuel from the secondary nozzle. The secondary nozzle is arranged to discharge fuel into a throat region between an upstream primary combustion chamber and a downstream secondary combustion chamber. In this preferred embodiment, fuel is supplied to the secondary nozzle through concentrically arranged diffusion and premix pipes.
The premix fuel supply pipe is connected to a centrally located premix fuel passage which extends axially along a center portion of the nozzle. It will be appreciated, of course, that: the fuel is not premixed with air prior to injection. Reference to the premix fuel supply pipe and/or premix fuel passage is merely intended to aid in understanding the invention and to maintain a convenient distinction between the fuel for the premixed mode of operation and the fuel for the diffusion mode of operation. The premix fuel passage includes an enlarged diameter portion and an interconnected smaller diameter pilot portion. A small portion of premix fuel is discharged through a single pilot orifice located at a forward end of the nozzle.
The secondary fuel nozzle also includes a plurality of radially outwardly extending gas injectors, each of which contains a number of fuel discharge orifices, the pipes extending radially outwardly from the premix passage, beyond the body portion of the secondary nozzle for discharging fuel into an area between the nozzle and the liner to mix with combustion air within the liner for discharge into the secondary combustion chamber.
The secondary nozzle is also provided with a series of radial air inlets which communicate with an air passage surrounding a portion of the premix fuel passage for discharging combustion air via a swirler located adjacent the premix fuel pilot orifice.
A diffusion fuel passage (reference to a diffusion fuel passage merely has reference to that portion of the fuel supply which will be used in the diffusion mode of operation) is provided within the secondary nozzle radially outwardly of both the premix fuel passage and the above described air passage for discharge through a plurality of orifices located in an annular array at the forward end of the secondary nozzle.
Thus, in accordance with an exemplary embodiment of the invention, there is provided a secondary nozzle for a gas turbine which includes primary and secondary combustion zones, the secondary nozzle comprising an elongated, tubular nozzle body having an inlet end and an outlet end; a tubular core assembly having an outer diameter less than an interior diameter of the tubular nozzle body to thereby define a first axial fuel passage between the core assembly and the tubular nozzle body extending to at least one discharge orifice at the outlet end, and a second axial fuel passage through the core assembly extending to a pilot orifice at the outlet end; an air passage located along at least a portion of the tubular nozzle body and located radially between the first and second fuel passages; and, a plurality of gas injectors extending radially out of the elongated tubular nozzle body from the second fuel passage, upstream of said outlet end.
Other objects and advantages of the subject invention will become apparent from the detailed description which follows.